The present invention relates to printers of the type in which characters are formed by means of tiny droplets of ink which are electronically caused to impinge directly on a recording medium (paper). More particularly, the present invention relates to an ink jet printing system in which ink droplets are expelled on demand from one or more nozzles by an electronically controllable mechanical force.
The rapidity of modern day data processing imposes severe demands on the ability to produce a printed record at very high speed. At the same time, the modern office environment places a high degree of importance on the need for a quiet printing operation. The thermal ink jet printer admirably meets these requirements. In the co-pending patent application of John L. Vaught, et al, entitled "Thermal Ink Jet Printer," Ser. No. 415,290 filed Sept. 7, 1982, now U.S. Pat. No. 4,490,728, and assigned to the instant assignee, a printing system is described which utilizes an ink-containing capillary having an orifice from which ink is ejected in the form of a tiny droplet. Ejection is caused by an ink-heating mechanism in the form of a thermal resistor located within the capillary and adjacent to its orifice. Upon the application of a suitable current to the resistor, the resistor is rapidly heated. A significant amount of thermal energy is transferred to the ink in the capillary resulting in the vaporization of a small portion of the ink adjacent to the orifice and producing a bubble in the capillary. The formation of this bubble in turn creates a pressure wave which propels a single ink droplet from the orifice and onto a nearby writing surface. By properly selecting the location of the ink-heating mechanism with respect to the orifice and with careful control of the energy transfer from the heating mechanism to the ink, the ink bubble will quickly collapse on or near the ink-heating mechanism before any vapor escapes from the orifice.
It will be appreciated that ink, by its nature, is a medium which is intended to dry relatively rapidly. This being so, it will be further appreciated that clogging of the tiny orifices in the print heat of a thermal ink jet printer is apt to occur, especially when one considers that the print heat contains a heat source, the heat from which can accelerate the evaporation of solvents containing ink pigments, often leaving behind a crust of solid ink particles. It will also be understood that capillary structures often require a "priming" action in order to initiate and maintain a continuous flow of ink into the capillary, much like a pump requires priming. The replacement of the ink supply, once it is depleted, likewise will require re-priming. Finally, the small orifices in the print head are subject to contamination by "dirt," which is usually in the form of extremely small dust particles from the adjacent recording medium. Almost all paper products, for example, have an accumulation or residue of fine paper particles or dust on them. Any one of the foregoing factors may cause an interruption of the supply of ink to the orifice or prevent the ejection of an ink drop therefrom.
In the co-pending application of Ross. R. Allen entitled "A Self-Cleaning Ink Jet Drop Generator having Cross Talk Reduction Features," Ser. No. 444,108 filed Nov. 24, 1982 and assigned to the instant assignee, a thermal ink jet printhead is disclosed in which the orifice plate is provided with a plurality of drain holes adjacent to the ink-ejecting orifices for the purpose of draining off any accumulations of ink on the outer surface of the orifice plate. Allen also mentions other methods for cleaning this surface, such as blowing away ink drops by air jets, wiping the surface of rolling an absorbent roller across the surface. These methods are characterized by Allen as requiring additional mechanisms or being intermittent in operation or being a source of contamination themselves, especially in the case of wipers or rollers. Intermittent operation is said to allow excess ink to accumulate and allowing residues to form.
Various other ways have been proposed to clean the surface of the member or plate containing the orifices from which ink is ejected usually by providing means such as shields or wipers for wiping such surface and/or flushing the orifices. Typical of these techniques are those disclosed in U.S. Pat. Nos. 4,112,435 to Kattner et al.; 4,177,471 to Mitchell; and 4,371,881 to Bork et al. In this latter patent an elastic closure plate, mounted on the inside of a protective cap or shield secured to the printhead carriage, is moved over the orifice during periods of non-use. For flushing, the closure plate is moved away from the orifice by partially rotating the entire cap assembly and ink, released by means of a pressure burst in the ink supply system, is conducted away from the writing head by means of the enclosing shield or cap against the inside of which the ink impinges. A resilient wiper blade is also provided on the inside of the cap/shield member adjacent the elastic closure plate. As the cap/shield member is partially rotated the wiper element sweeps over the jet openings to sweep away any dirt or unwanted debris. The Kattner patent discloses a combination protective shield and cleaning wiper mounted on a printhead which is movable to and from a blocking position in front of the orifices, the wiper blade or arm wiping the orifice face during movement between positions. The Mitchell patent discloses a cap member of urethane plastic for engagement with his printhead to form a seal therewith when the printhead is moved toward the cap member by a cam arrangement. The housing for the cap member also includes a doctor's blade and a pad wet with glycerine to clean and wipe any ink off the cap member itself. Also provided is a purge needle to remove ink flushed through the orifices, this needle being located in front and to the side of the cap and being connected to a purge pump.